The present invention relates to a small-sized zoom lens having a high magnification ratio and high efficiency, being thereby suitable for a video camera.
In recent years, development of zoom lenses for various video cameras has become active with popularization of a homestyle video camera of an integration type. Zoom lenses having a high magnification ratio such as 8 times or more are known, and as the small-sized ones among them, there are known zoom lenses such as those described in Japanese Patent Publication Open to Public Inspection No. 13109/1992 (hereinafter referred to as Japanese Patent O.P.I. Publication) wherein five lens groups having respectively refractive powers which are positive, negative, positive, positive and positive in that order from the object side are incorporated, and the second and fourth lens groups move in the course of zooming. In a zoom lens of this type, the number of moving lens groups including that for focusing is two, owing to the focusing function of the fourth lens group, which is a special feature advantageous for making a zoom lens small. In the case of a zoom lens of this type, however, it has been difficult to achieve a high magnification ratio and high efficiency.
Further, when a long focal length is taken at a telescopic end for achieving a high magnification ratio, the amount of movement of the fourth lens group for focusing at the telescopic end is increased, and aberration variation between the infinite object distance and the closest object distance is large. When a short focal length is taken at a wide angle end position to avoid the problem mentioned above, an increase of distortion is brought about, which is disadvantageous. Further, to satisfy the demand for high efficiency of a zoom lens, it has been considered to use many aspheric surfaces in the zoom lens. However, when an aspheric surface is used for a lens having a strong refractive power in a zoom lens, efficiency of the zoom lens is extremely deteriorated, which has been a problem.
Due to the problems mentioned above, zoom lenses known so far have not necessarily been satisfactory to obtain a small-sized zoom lens having a high magnification ratio and high efficiency.